Aswin has created an interesting NXT gear box that "uses two NXT motors both for driving the output and for shifting gears. As such the full power of the two motors is available for propelling a device while there is no need for a dedicated motor for shifting."
You can see the device in action below. A detailed video explanation is here, and pictures can be found here.
You can see the device in action below. A detailed video explanation is here, and pictures can be found here.